Excel Spreadsheet Problem

Tom, who is age 35 and single, is not covered by a pension plan at work. He is currently in the 25% marginal tax bracket. He plans to retire at age 70. Should he make annual contributions to a regular IRA or a Roth IRA? He expects to earn annual rate of return on his contributions of 4%, expects to be in the 25% bracket during his employment years, and expects to be in the 15% bracket during his retirement years. [ Hint: assume the contributions on the Roth account equals the contribution to the regular IRA x (1-marginal tax rate).] How would your answer change if he expects to be in the 30% marginal tax bracket when he retires?